1. Field of the Invention
The present invention relates to fine nickel powder and a process for producing it, more specifically an efficient process for producing fine nickel powder, capable of metallizing a nickel compound particles at low temperature to prevent its sintering, and fine nickel powder produced by the process, having a flat shape, a narrow size distribution and uniform thickness, and suitable for various applications such as internal electrodes for laminate ceramic capacitors of high electric capacity.
2. Description of the Prior Art
Fine nickel powders have been traditionally used as electro conductive pastes material for electrical circuit devices, such as electrodes for laminate ceramic parts, e.g., laminate ceramic capacitors and multilayered ceramic substrates, in which the material is used as an electroconductive material for thick films. Recently, the used laminate ceramic capacitors as electric component has grown. A laminate ceramic capacitor is composed of ceramic dielectrics and internal metallic electrodes, alternately stacked in layers, fast pressed to each other, and sintered to form a monolithic structure. For production of internal electrodes, a mixture of nickel powder and an organic vehicle, with an organic binder (e.g., cellulosic resin) dissolved in a solvent, is kneaded by a 3-roll mill or the like. Then, the resulting electroconductive paste is placed on a green sheet of ceramic dielectric (e.g., barium titanate) by printing, and the laminate is cut to a given size and sintered in a neutral or reducing atmosphere to produce the internal electrodes.
With the progress of miniaturization of electronic components, demand of ultrafine metal particles is growing. In the market of multilayered ceramic capacitors, one of the largest electronic components demand is thinning of metal electrode layers. Discontinuity of metal electrode layers occurs when the layer becomes thinner because of the decrease in the number of stacking particles. If the size of particles becomes smaller, the stacking number will increase. However, coagulation of particles in paste will be the problem with decreasing particle size, resulting in discontinuity of metal electrode layer. Furthermore, when the metal particle size decreases to below 0.1 μm (100 nm), handling of these products becomes difficult. Moreover, the temperature at which these particles are sintered will sharply decrease to cause problems, e.g. breaking of films of these particles.
Because of the problem mentioned above, a new approach producing a new material is required. Obviously, in this regard, we are approaching the limit of thickness of metal electrode.
However, by controlling the shape of metal particles, it may be possible to reach nanometer in the thickness and sub-micrometer in longitudinal direction, leading to achieving the desired properties for MLCC. For example, it should be possible to produce a dramatic increase in stacking number by the use of nanometer metal flake with keeping easy handling performance by using shape-controlled particles, such as Ni flakes. Thus, with the platelet-like particle of 0.2˜0.3 μm in diameter and 20˜30 nm in thickness, it should be possible to produce high performance of metal electrode layers because the stacking number are tenfold.
Both physical and chemical processes for producing fine powder of metallic nickel have been reported. One of the physical processes is based on deformation of metals by attrition. For example, JP-A 2004-84055 (Pages 1 and 2) discloses that attrition of nickel particles gives flaked particles having an average thickness of 0.03 to 0.5 μm. However, particles varying so much in diameter cannot give internally very thin electrodes in a reproducible manner. Obviously, this process cannot yield metallic particles of properties that are required in applications described above.
The chemical processes are essentially based on reduction of the metal ions in solution. By growing certain faces of metal crystals to produce fine particles with desired shapes, substance is adsorbed on a crystal plane to retard crystal growth of certain plane and by yielding particles of anisotropic shape. For example, Nano Letters, vol. 13, no. 5, 2003, p. 675-679 discloses the synthesis of triangular plate shape silver particles in an aqueous solution by using the described method. Although this process is giving the triangular plates having a diameter of around 100 nm, it is not practical due to low silver concentration.
Another chemical process is based on hydrogen-aided reduction of a metal hydroxide, oxide or, the like, because the latter can be more directly obtained in a different morphological shape than pure metal. For example, Langmuir, vol. 4, 1988, p. 26-31 discloses a technique by which iron oxide particles coated with silica was reduced to pure iron, thus preventing sintering of the particles during the reduction process. However, in this case, silica coating prevents production of conducting metal paste. Therefore, it is not suitable for the production of metallic nickel powder for laminate ceramic capacitors.
In another chemical process, it is proposed to synthesize nickel particles of flat shape (refer to, e.g., JP-A 11-152505 (Pages 1 and 2)) by reducing a mixture of at least one species of alkali-earth metal salt selected from the group consisting of the oxides, carbonates or hydroxides and nickel hydroxide with hydrogen at 800 to 1300° C. and then by dissolving alkali-earth metal salt in an acid. In this process the alkali-earth metal salt works as a barrier to diffusion of the metallic nickel particles and thereby prevents their growth. However, it is very difficult to completely prevent particle sintering because of inhomogeneous coating of the salt and high reduction temperature of 800° C. or higher, resulting in a product of flat shape, but in a variety of thicknesses from 0.05 to 0.9 μm.
In summary, it has been impractical to produce two-dimensional plate-shape Ni particles to mass produce for various applications such as internal electrodes for laminate ceramic capacitors.